There are several reasons that ink-jet printing has become a popular way of recording images on various media surfaces, particularly paper. Some of these reasons include low printer noise, capability of high-speed recording, and capability of multi-color recording. Additionally, these advantages can be obtained at a relatively low price to consumers. Though there has been great improvement in ink-jet printing technology, there is still improvement that can be made in many areas.
With respect to ink-jet ink chemistry, the majority of commercial ink-jet inks are water-based. Thus, their constituents are generally water-soluble, as in the case with many dyes, or water dispersible, as in the case with pigments. Furthermore, ink-jet inks typically have low viscosity to accommodate high frequency jetting and firing chamber refill processes common to ink-jet pens.
In addition to general image fade that occurs in many ink-jet ink systems, ink-jet prints have also been known for poor durability when exposed to water or high humidity. This results from the use of water-soluble and water dispersible colorants within the water-based ink. There has been great improvement in the area of water durability of ink-jet inks through incorporation of certain ink-jet compatible latexes. The latex can comprise small micron or submicron hydrophobic polymeric particles of high molecular weight that are dispersed in an aqueous fluid, which fluid ultimately becomes at least part of a liquid vehicle of an ink-jet ink. When printed as part of an ink-jet ink, latex particulates of the ink can form a hydrophobic print film on a media surface, entrapping and protecting the colorant within the film. However, such latex compositions can create problems with respect to pen reliability, as well as with respect to settling of the latex over time.
Conventional latex particles are normally designed to flocculate so that latex precipitate may be easily shaken or stirred back into dispersion without agglomeration. Such flocculation behavior is well known with latex paints. Unfortunately, these conventional teachings do not address the unique needs of ink-jet printing applications. For example, it would be desired that a latex for use in an ink-jet printing application have good dispersion stability, and thus, would tend to avoid flocculation while within the ink-jet architecture. The micro-channel ink feeds in ink-jet pens are easily clogged with precipitant, particularly when a pen is stored or otherwise unused for prolonged periods of time. Such precipitation is not easily redispersed by pen shaking, as flow constriction prohibits adequate mixing within micro-channels of pen architecture. Additionally, micro-channels used for jetting can house some of the ink over prolonged periods in preparation for firing, and settled latex can cause further constricting of the micro-channels. This can result in ink-jet pen failure due to clogging of the micro-channels. The micron-order settling distances found in the fluid channels of thermal inkjet pens exacerbate the problem. Additionally, latex particles in the firing chamber of a pen are subjected to explosive thermal shear conditions. Because of this, greater inter-particulate repulsion can be greatly desired.